He showed up once for me when I had 1 deity and unlocked then never appeared again. After I'd unlocked four deities, he appeared a second time and now he's gone again. I suspect what the devs had in mind was that the Pactmaker would appear repeatedly over the course of the game as you unlock other deities, and finally add himself to the pantheon to complete it. This would be lost on us since in our original progression the Pactmaker was added after we'd already unlocked everyone else.